The present invention relates to an optical distance meter, and particularly to a light-wave distance meter which has a light source having multiple spatially-separated light emitting areas for producing a light beam and has an extended maximum measurable distance based on the low coherence and high luminous intensity light source. The invention also relates to a light source device used suitably for the light-wave distance meter.
Conventional light-wave distance meters are designed to radiate a light beam produced by a light emitting diode (LED) to a corner cube prism that is the target of measurement, receive a returning echo light pulse from the corner cube prism, and calculate the distance to the target based on the time length between the light beam radiation and the return light pulse reception. Each distance meter has its maximum measurable distance determined from the luminous intensity of light source, which is two to three kilometers with the LED light source.
For extending the maximum measurable distance, a conceivable scheme is the use of a high-luminance semiconductor laser. A semiconductor laser of the continuous light emission type is comparable in size and power consumption with the conventional LED and allows direct modulation by the drive current, and therefore it can readily take the place of the LED. Moreover, the continuous semiconductor laser has another advantage of the faster light emission response as compared with the LED, and thus it has a potential capability of accomplishing a light-wave distance meter with enhanced accuracy based on the higher modulation frequency.
A semiconductor laser of the pulsed light emission type has a large peak power, allowing the extension of the maximum measurable distance and the non-prism measurement without using a corner cube prism, although the principle of measurement is different from the distance meter using the continuous semiconductor laser.
However, the above-mentioned conventional light-wave distance meter, either based on the continuous or pulsed semiconductor laser, has a problem of the narrower light emission spectrum and higher coherence as compared with the light-wave distance meter using LED.
The light-wave distance meter is required to have a resolution of distance measurement of 1 mm or less with respect to the modulation frequency or emission pulse width of the light source, which is equivalent to the time difference of the light beam radiation and reception of: EQU 1/C.times.2=6.6 psec (1)
where C is the velocity of light.
Assuming that the oscillation frequency is 15 MHz and the continuous modulation is of 50% duty-cycle, the light emission pulse width of the light source is: EQU 1/15.times.0.5=33 nsec (2)
For the distance meter based on the semiconductor pulse laser, a practical emission pulse width is around 20 nsec due to the limit of the drive circuit and element characteristics.
On this account, the conventional light-wave distance meters necessitate the interpolation process for determining the fundamental frequency component of the modulated light or the centroid of the emitted light pulse.
A high-coherence light source can cause the emergence of adverse interference in various sections of the optical system, e.g., the speckle in the multi-mode optical fiber, uneven near field pattern and far field pattern attributable to the speckle in the interior of the multi-mode optical fiber and the speckle at the outlet of the multi-mode optical fiber, the variation of speckle due to the mode jump of the emission wavelength, and the variation in the modulated waveform in the case with the application of luminous modulation.
The light-wave distance meter is vulnerable to the emergence of uneven waveforms attributable to the speckle of the light emission optical system, the variation of speckle caused by the mode jump of the light source, and the emergence of the spatial unevenness of waveform due to the variation of interference of the emitted light caused by the movement of atmosphere. These uneven waveforms shift the centroid of the modulated waveform, resulting in a degraded accuracy of distance measurement.
Although it is necessary to lower the coherence of the light source, the semiconductor laser has its spectral width determined from its structure, and the coherence of the light source cannot simply be diminished.